1. Field of the Invention
The present invention relates to a composite material, which is composed of a ceramic component and a metallic component, and a method for manufacturing the composite material and a wafer holding member including the composite material. The present invention also relates to a wafer holding member for holding a wafer, such as semiconductor substrate or a substrate for liquid crystal, either in a deposition process, such as CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), sputtering, SOD (Spin-on Dielectric), SOG (Spin-on Glass), or in an etching process, and a method for manufacturing the wafer holding member.
2. Description of the Related Art
A composite material composed of a metal and a ceramics has characteristics that a coefficient of thermal expansion can be optionally adjusted by changing the ratio of metal to ceramics, with a desired thermal conductivity by selecting metallic species having a higher thermal conductivity and ceramic species. In a heat dissipation substrate for an IGBT (Insulated-Gate Bipolar Transistor) for example, in order to remove heat emitted from an IC chip, which is soldered to a Cu-metalized film on a ceramic substrate, the above-mentioned composite material is joined to the back of the ceramic substrate with metal adhesives or the like so as to function as a heat sink.
In addition, some composite materials are increasingly developed in the field of semiconductor manufacturing apparatus. In a semiconductor wafer process for manufacturing a semiconductor device, e.g., in either a deposition process, such as CVD, PVD, sputtering, or an etching process, it is important to deposit a homogeneous film having a uniform thickness on an objective wafer and to form an etching having a uniform depth on the deposited film. Therefore, a wafer holding member, which can facilitate temperature control of the wafer, is used. For the wafer holding member, for example, an electrostatic chuck includes a plate base, one principal plane of which serves as a placing surface for a wafer; and an internal electrode for attraction, which is provided near the placing surface of the plate base, wherein the wafer is held on the placing surface by generating an electrostatic attracting force between the wafer on the placing surface and the internal electrode.
Further, there is another type of wafer holding member which can heat up the wafer, where another internal electrode for heating is provided in the vicinity of another principal plane of the plate base. Both the internal electrodes for attraction and heating are electrically connected to feeding terminals, respectively. When the wafer is placed on the placing surface and a voltage is applied to the feeding terminals, the electrostatic attracting force is generated between the wafer and the attracting electrode so that the wafer is firmly held on the placing surface. Coincidentally, the wafer can be also heated up to a high temperature.
In yet another type of wafer holding member, the undersurface of which is joined with a metal plate, applying an RF voltage between the plate and an opposite electrode (not shown) can efficiently generate plasma above the wafer.
Recently, for internal wiring in a semiconductor device, conventional aluminum wiring is gradually shifted to copper wiring. In the copper wiring, heating a wafer up to a high temperature is not required. A wafer holding member having an electrostatic attracting function of holding the wafer around a room temperature is needed. The wafer mounted on the placing surface of the wafer holding member is exposed to plasma, including Cu or Ar, with the temperature rising up. In order to suppress the rise in temperature, there is a cooling approach where a plate, which is formed of a composite material having a higher thermal conductivity of 150 W/(m·K) or more, either composed of Al and SiC or composed of Al, Si and SiC, is joined to the wafer holding member with a brazing material or solder, so as to remove heat from the heated wafer subjected to plasma, including Cu or Ar, by water-cooling or air-cooling the plate. Such a plate, formed of a composite material, joined to the wafer holding member as used under the condition as described above requires the thermal conductivity of 160 W/(m·K) or more, the coefficient of thermal expansion close to that of the ceramics used in the wafer holding member, and the He leak rate of 1.3×10−10 Pa·m3/sec or below.
The prior document 1 (Japanese Patent Unexamined Publication (kokai) JP-A-10-32239 (1998)) proposes an electrostatic chuck, as shown in FIG. 4, including a plate base 24 of ceramics, in which an electrode 20 for electrostatically attraction is embedded, and a plate 23 composed of ceramics and Al, which is joined to the plate base 24. SiC is proposed as a ceramic component included in the plate 23. A approach where the plate base 24 and the plate 23 are joined to each other using a brazing material or solder by selecting a joint temperature in a range of 150 to 630 degree-C. corresponding to the ratio of the ceramic component in the plate 23 is also proposed.
The prior document 2 (Japanese Patent Unexamined Publication (kokai) JP-A-15-155575 (2003)) proposes another approach where a plating film is provided on a surface of a composite material composed of aluminum and SiC for joining other objects. Each plate described in the prior document 1 or 2 is composed either of ceramics and Al or of SiC and Al, and basically porous. Because of the difference in coefficient of thermal expansion between the fused metal and the ceramics like SiC, pores are created in cooling process after impregnating a ceramic preform with the fused metal or after casting a mixture of the fused metal and the ceramic into a mold, based on the difference in contraction due to the difference in coefficient of thermal expansion between the metallic species and the ceramic species.
The prior document 3 (Japanese Patent Unexamined Publication (kokai) JP-A-3-3249 (1991)) proposes yet another approach where a water-cooled electrode formed of Al and an electrostatic chuck formed of ceramics are joined to each other on a joint plane having a plating of In film at a temperature of 170 degree-C. or below.
The prior document 4 (Japanese Patent Unexamined Publication (kokai) JP-A-2003-37158) proposes an electrostatic chuck, as shown in FIG. 5, including a base 34, an insulating layer 36 formed on the base 34, an electrode 37 provided thereon, and a dielectric layer 38 provided for covering the electrode, characterized in that a metal layer 35 is formed on the surface of the base 34, and the base 34 is formed of a metal-ceramic composite material compounded of metal and ceramic powder, and the dielectric layer 38 has a volume resistibility value of 1×108 to 5×1013 ohm·cm.
The prior document 5 (Japanese Patent Unexamined Publication (kokai) JP-A-2003-80375) proposes a wafer holding member in which a metal member and a ceramic member are joined to each other via a metal layer.
However, since the above-mentioned composite plate, composed either of two components of Al and SiC or of Al, Si and SiC, is porous itself, when the plate base serving as a wafer holding member and the plate are joined to each other using a brazing material or solder, it cannot maintain vacuum gas-tightness of 1.3×10−10 Pa·m3/sec or below in terms of He leak rate, which is required for a semiconductor manufacturing apparatus.
The wafer holding member used for a leading-edge semiconductor manufacturing process requires the He leak rate of 1.3×10−10 Pa·m3/sec or below, the Weibull modulus, indicating variation in strength of the material, of 5 or more, the thermal conductivity of 160 W/(m·K) or more, and the coefficient of thermal expansion of 4×10−6 to 6×10−6/degree-C., close to that of the ceramics. The conventional composite material cannot simultaneously satisfy all the factors of He leak rate, variation in strength, thermal conductivity and coefficient of thermal expansion.
Further, a higher thermal conductivity is needed for removing efficiently heat of the Si-wafer heated by plasma. A wafer holding member, in which a plate base and a plate of Al are joined to each other using Al-brazing or In-brazing in order to increase the thermal conductivity, has a problem that the plate base is cracked under a thermal shock cycle of −40 to 100 degree-C., which is required for a deposition apparatus, such as CVD, PVD and sputtering, or an etching apparatus, because of the large difference in coefficient of thermal expansion between the plate of Al and the plate base.
Furthermore, when the composite plate, composed of two components of aluminum and SiC, and the ceramic plate are joined to each other using metal, wetting between the aluminum and the metal joint layer is not so good. Even though the composite plate of two components and the ceramic plate are integrated using metal joint, pores are likely to be created in the joint interface because of the bad wetting between the metal joint layer and the aluminum. The resulting pores cannot ensure the reliability under the thermal shock cycle of −40 to 150 degree-C., which is required for a deposition apparatus, such as CVD, PVD, sputtering, SOD or SOG, or an etching apparatus, even using the conventional manufacturing method as proposed in the prior document 1 (JP-A-10-32239) or 4 (JP-A-2003-37158).
Since the deposition apparatus, such as CVD, PVD, sputtering, SOD or SOG, or the etching apparatus employs a process in vacuum, a placing surface 20a of the wafer holding member is exposed to a high vacuum and a composite plate 23 is used in the ambient air.
Then, a coolant gas, such as Ar, is introduced through a gas feeding hole 21. A joint material 22 is generally subject to a kind of thermal shock cycle of −40 to 150 degree-C. Therefore, the reliability of the joint material 22 is required.
In FIG. 4, showing the conventional example, when using for an electrostatic chuck, some gas species is introduced to the back of a Si wafer through the gas feeding hole 21. The joint material 22 requires the vacuum gas-tightness as much as 1.3×10−10 Pa·m3/sec in terms of He leak rate. The composite plate is composed of two components of aluminum and SiC. In case metal is used for the joint material, wetting between the aluminum and the metal joint material is bad, resulting in minute pores created between the metal joint material and the aluminum. It is difficult to maintain the reliability under the thermal shock cycle.
Further, after depositing a desired film on the wafer attached onto the placing surface using CVD, PVD, sputtering, SOD or SOG in a vacuum chamber or etching, it is necessary to release the wafer from the placing surface immediately to proceed the next process. But there is a problem on residual attracting force, that a force for attracting the wafer remains because electric charges, which have been accumulated on the placing surface 20a after applying voltage to the electrode 20 for electrostatic attraction to attract the wafer to the placing surface 24a and then turning off the applied voltage to release the wafer on completion of a deposition or etching process, is not neutralized immediately. While the residual attracting force emerges, there are some problems that the wafer cannot be released even in an effort to put the wafer off the placing surface 20a, positioning of the wafer is out of order if released, or at its worst, the wafer is broken when the wafer is forced to be released form the placing surface.
Furthermore, there is a possibility that the residual attracting force emerges after repeating about 10,000 times a step for applying voltage to the electrode for electrostatic attraction through feeding terminals to attract the wafer W by generating an electrostatic attracting force, a step for depositing or etching on the wafer W in a vacuum chamber, and a step for turning off the voltage applied to the feeding terminals to release the wafer W on completion of depositing or etching.